1. Field of the Invention
The present invention relates generally to image sensors, and more particularly, to a metal-oxide semiconductor (MOS) image sensor having source-follower transistors operating in saturation for a wide operating range.
2. Background of the Invention
Generally, image sensors convert an optical image into electric signals. With developments in the computer and communication industries, image sensors are in high demand especially with improved performance, such as in digital cameras, camcorders, personal communication systems, gaming devices, security cameras, medical micro-cameras, or robots, for example.
Metal oxide semiconductor (MOS) image sensors are easy to drive and may be driven with various scanning techniques. Additionally, the MOS image sensor with a signal processor is easily integrated into a single chip having miniaturized size and reduced manufacturing cost using typical MOS fabrication processes. The power consumption of the MOS image sensor is relatively low such that the MOS image sensor is amenable for application in devices with limited battery capacity. Accordingly, use of MOS image sensors with high resolution has dramatically increased.
Each pixel in a MOS image sensor includes a photoelectric converter that generates electric charge in response to received light. Such electric charge varies the voltage of a detection node. In addition, each pixel includes a source-follower transistor that is coupled to the detection node for generating an output signal indicating the amount of electric charge transferred to the detection node. Such a source-follower transistor is desired to operate in the saturation region.